Conventionally, a mobile information terminal device used in a liquid crystal display device of a mobile phone or the like is driven by a buttery, so that a significant object is to reduce its power consumption. As to the reduction of power consumption of the information terminal device, a technique for decreasing a refresh rate (refresh cycle) is known. The technique for decreasing a refresh rate is described as follows with reference to drawings. Note that, the refresh rate means “how often a screen on the display is switched (updated)”. In case where the refresh rate is 60 Hz, the screen is switched 60 times per second.
FIG. 20(a) is a timing chart illustrating the case where the refresh rate is 60 Hz. FIG. 20(a) shows a vertical synchronization signal (Vsync), a horizontal synchronization signal (Hsync), a dot clock (dot CK), and a video data signal (Video). Note that, a single vertical scanning period (1V) is 16.7 mS, a horizontal scanning period (1H) is 25 μS, a dot CK is 48 MHz, and 1V is 660H. The vertical scanning is carried out so as to correspond to a timing of the vertical synchronization signal, so that a frequency of the vertical synchronization signal serves as the refresh rate. In this manner, when the refresh rate is 60 Hz, the screen is switched 60 times per second, so that power consumption increases. Therefore, a technique in which the refresh rate is decreased to 40 Hz for reduction of power consumption is conventionally adopted.
FIG. 20(b) is a timing chart illustrating a case where the refresh rate is 40 Hz. As in FIG. 20(a), also FIG. 20(b) shows a vertical synchronization signal (Vsync), a horizontal synchronization signal (Hsync), a dot clock (dot CK), and a video data signal (Video). Note that, a single vertical scanning period (1V) is 25.0 mS, a horizontal scanning period (1H) is 38 μS, a dot CK is 32 MHz, and 1V is 660H. That is, the frequency of the dot CK is decreased and the single vertical scanning period is increased so as to decrease the refresh rate, thereby driving liquid crystal more slowly.
FIG. 21 is a graph illustrating a relationship between a refresh rate and power consumption. A vertical axis indicates power consumption [mW] and a horizontal axis indicates a refresh rate [Hz]. As shown in FIG. 21, when the refresh rate is 60 Hz, the power consumption is 452 mW. When the refresh rate is 40 Hz, the power consumption is 368 mW. In this manner, it is possible to reduce power consumption about 19%.
However, a high-speed refresh rate may be required depending on an image to be displayed. As to this case, each of Patent Document 1 and Patent Document 2 describes a technique for switching a refresh rate.
More specifically, Patent Document 2 discloses the following technique. In case of using the information terminal device as a mobile phone, a high-speed refreshing operation (operation at a refresh rate of 60 Hz) is carried out in a normal display state such as a phone-call state and a low-speed refreshing operation (operation at a refresh rate of 40 Hz) is carried out in a bare essential display state such as a standby state.
[Patent Document 1]
Japanese Unexamined Patent Publication Tokukai 2002-123234 (Publication date: Apr. 26, 2002)
[Patent Document 2]
Japanese Unexamined Patent Publication Tokukai 2002-116739 (Publication date: Apr. 19, 2002)
[Patent Document 3]
Japanese Unexamined Patent Publication Tokukaihei 10-10489 (Publication date: Jan. 16, 1998)